Micro-electro-mechanical system (MEMS) devices, in particular inertial sensors such as accelerometers and angular rate sensors or gyroscopes, are being used in a steadily growing number of applications. Due to the significant increase in consumer electronics applications for MEMS sensors such as optical image stabilization (OIS) for cameras embedded in smart phones and tablet PCs, virtual reality systems and wearable electronics, there has been a growing interest in utilizing such technology for more advanced applications which have been traditionally catered to by much larger, more expensive and higher grade non-MEMS sensors. Such applications include single and multiple-axis devices for industrial applications, inertial measurement units (IMUs) for navigation systems and attitude heading reference systems (AHRS), control systems for unmanned air, ground and sea vehicles and for personal indoor GPS-denied navigation. These applications also may include healthcare, medical and sports performance monitoring and advanced motion capture systems for next generation virtual reality. These advanced applications often require lower bias drift and higher sensitivity specifications well beyond the capability of existing consumer-grade MEMS inertial sensors on the market. In order to expand these markets and to create new ones, it is desirable and necessary that higher performance specifications be developed. It is also necessary to produce low cost and small size sensor which can be integrated in MEMS inertial sensor-enabled system(s).
In particular, there has been increasing interest in using advanced MEMS inertial sensors (accelerometers and gyroscopes) to develop low-cost miniature Inertial Measurement Units (IMUs) for navigation systems (i.e. Inertial Navigation Systems or “INS”) and Attitude Heading Reference Systems (AHRS) for unmanned air, ground and sea vehicles. There has also been a growing need to develop both military and commercial grade personal navigation systems (PNS). MEMS accelerometers and gyroscopes, being much smaller than traditional mechanical gyroscopes, are subject to higher mechanical noise and drift. Since position and attitude are calculated by integrating the acceleration and angular rate data, the noise and drift lead to growing errors in position. Consequently, for navigation applications, it is desirable to augment the MEMS 6DOF inertial capability (3 axes of acceleration and 3 axes of angular rotation) with pressure and other measurements via sensor fusion. Pressure can provide altitude information which can be used as a check against MEMS drift in order to “re-zero” the error.
As is known in the art, a pressure sensor converts a difference in pressure into a variation in an electrical quantity such as capacitance or resistance. Miniature pressure sensors fabricated with semiconductor or MEMS technology chiefly consist of two types: capacitive and piezoresistive. A pressure sensor typically consists of a thin flexible membrane suspended over a cavity that is evacuated (for absolute pressure measurements) or filled with a gas at some fixed pressure (for relative pressure measurements). A pressure difference across the membrane causes it to deflect. The deflection can be measured by placing piezoresistors at the edge of the membrane and measuring the change in resistance as taught by U.S. Pat. No. 6,417,021 B1 or U.S. Pat. No. 8,468,888 B2 for example. Alternatively, the deflection can be measured by monitoring the capacitance formed by the membrane and the bottom of the cavity as taught by U.S. Pat. No. 8,316,718 B2 or U.S. Pat. No. 6,743,654 B2 for example. Capacitive sensors are increasingly popular because they consume less power than piezoresistive sensors.
In order to improve the performance of MEMS IMUs, a pressure sensor can be added by using commercial off-the-shelf (COTS) sensors placed on the IMU board or package substrate with the inertial sensors, or by stacking them on the MEMS inertial sensor die to produce a System-In-Package or “SIP”. However, with either approach, additional lateral or vertical board or chip space is required to accommodate the footprint of the pressure sensor, as well as additional wire bonding or integrated circuit (IC) soldering to establish electrical connections with the pressure sensor and an external integrated circuit (IC) or printed circuit board (PCB) in order to read the pressure sensor signal.
There is thus need for an improved MEMS pressure sensor and manufacturing method.